1. Field of the Invention
The present invention relates to a solid polymer electrolyte membrane type fuel cell and to a fuel cell stack constituted by stacking a plurality of the fuel cell units, and more specifically, relates to a technique for absorbing expansion and contraction of the fuel cell stack in the stacking direction of separators.
2. Description of the Related Art
Fuel cells include a solid polymer electrolyte membrane type fuel cell constituted by providing a pair of electrodes on opposite sides of the solid polymer electrolyte membrane and sandwiching the outside thereof by a pair of separators.
In this fuel cell, a passage for a fuel gas (for example, hydrogen) is provided on the entire surface of a separator provided facing one electrode, a passage for an oxidant gas (for example, air including oxygen) is provided on the entire surface of a separator provided facing the other electrode, and a passage for a cooling medium is provided on either one of the surfaces of separators opposite to a surface facing the electrode.
When the fuel gas is supplied to the reaction surface of one electrode, hydrogen is ionized and moves to the other electrode via the solid polymer electrolyte membrane. Electrons generated during the reaction process are taken out to an external circuit, and are used as direct-current electrical energy.
Since the oxidant gas is supplied to the other electrode, the hydrogen ions, the electrons and the oxygen react with each other to thereby generate water.
The surface on the opposite side of the electrode reaction plane of the separator is cooled by the cooling medium flowing between the separators.
Since these reactant gases and the cooling medium should flow in respectively independent passages, a sealing technique, which separates each passage, is important.
The portions to be sealed include, for example, the peripheries of communication holes formed penetrating through the separator so as to distribute and supply the reactant gas and the cooling medium to each fuel cell unit in the fuel cell stack, the outer peripheries of membrane electrode assembly formed of the solid polymer electrolyte membrane and a pair of electrodes arranged on opposite sides thereof, the outer peripheries of a coolant passage plane of the separator, and the outer peripheries of front and back faces of the separator. As the sealing material, an elastic and adequately resilient material, for example, an organic rubber, is adopted.
Conventionally, a fuel cell having a membrane electrode assembly by sandwiching a solid polymer electrolyte membrane by a pair of electrodes and sandwiching the outside thereof by a pair of separators, comprises a membrane electrode assembly (as shown in FIG. 30) constituted by sandwiching a solid polymer electrolyte membrane having a larger outer size between a pair of gas diffusion layers each having the same size, and the outer size thereof is smaller than that of the solid polymer electrolyte membrane, as disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 8-148169. In this type of fuel cell 40, the passage for the fuel gas 41 and the passage for the oxidant gas 42 are sealed by sandwiching with a pair of O-ring the portion of the solid polymer electrolyte membrane 45, which is protruded from the outer peripheries of the anode electrode 43 and the cathode electrode 44. However, in such a sealing structure, a problem arises in that sealing of passages may fail if a pair of O-rings each disposed on both side of the solid polymer electrolyte member are insufficiently aligned.
For example, as shown in FIG. 31, if two O-rings on both surface of the solid polymer electrolyte membrane are disposed out of positions, the pressure of both O-rings press the solid polymer membrane and the solid polymer electrolyte membrane 45 is be deformed such that the surface pressure of the O-rings becomes insufficient to provide a hermetic seal. In addition, an unfavorable phenomenon will be caused by deformation of the solid polymer electrolyte membrane in that the solid polymer electrolyte membrane will be peeled off from the anode electrode 43 and the cathode electrode 44.
In order to avoid such unfavorable phenomena, the grooves to align O-rings must be formed in a very precise manner, which results in increasing the manufacturing cost.
Since the fuel cell 40 is used as a fuel cell stack after stacking a plurality of fuel cell units, the thickness of the fuel cell unit is desired to be as thin as possible.